Recently, as communications systems have developed, much notice is paid to wireless transmission of Hi-Vision image signals. High-Vision image signals must transmit large amount of information and accordingly wireless transmission devices using millimetric waves that assure wide band have been developed.
A wireless transmission device includes, for example, a high frequency circuit for converting a transmission signal into a high frequency signal; and an antenna for transmitting the high frequency signal as radio waves to an opposite communication device.
However, in a case of a wireless transmission device using millimetric waves, separately providing a high frequency circuit and an antenna and connecting them cause such a problem that electricity is lost greatly at a point where the high frequency circuit and the antenna are connected with each other.
In order to reduce the loss of electricity at the point where the high frequency circuit and the antenna are connected with each other, integrated antenna modules have been developed. An integrated antenna module contains a high frequency circuit and an antenna in one module.
An example of the integrated antenna modules is disclosed in Japanese Unexamined Patent Publication No. 237867-1997 (Tokukaihei 9-237867; published on Sep. 9, 1997) (hereinafter referred to as Document 1). This integrated antenna module is explained below with reference to FIG. 9.
FIG. 9 is a cross sectional drawing illustrating a structure of a conventional integrated antenna module.
As shown in FIG. 9, the integrated antenna module includes: an antenna circuit substrate X in which an antenna element 902 and a high frequency line 903 for supplying a current to the antenna element 902 are formed on a first dielectric substrate 901; and a high frequency substrate Y in which a high frequency device 906 is contained in a cavity 905 formed in a part of a second dielectric substrate 904 and the high frequency device 906 is sealed by a lid member 907, and a transmission line 908 for transmitting a signal to the high frequency device 906 is formed. The antenna circuit substrate X and the high frequency substrate Y are integrally laminated. Further, the antenna circuit substrate X and the high frequency substrate Y include, as their internal layers, a grand layer 909 and a grand layer 910, respectively.
However, in Document 1, although much of high frequency signals generated by a high frequency circuit are irradiated as radio waves from the antenna element 902, a part of the high frequency signals is propagated as surface waves on a surface of the antenna circuit substrate X where the antenna element 902 is mounted, and the surface waves are irradiated from ends of the antenna circuit substrate X.
Consequently, as the size of the integrated antenna module is made smaller for reducing costs, surface waves irradiated from the ends of the antenna circuit substrate X increase. As a result, a radiation pattern of radio waves irradiated upward from the antenna element 902 is influenced by the surface waves and the radiation pattern is changed.
In the worst case, the antenna element 902 has very low antenna gain at an area above the antenna element 902. Consequently, if the antenna element 902 has, at the area above it, a radiation angle at which antenna gain is very low, a little difference in an angle at which the wireless communication device is positioned may make communications disabled.